1. Field of the Invention
This invention relates to a wide-range air-fuel ratio sensor for an engine, and more particularly to a wide range air-fuel ratio sensor for detecting the air-fuel ratio in intake gas introduced into the engine by way of the oxygen concentration in the engine exhaust.
2. Description of the Prior Art
As is well known in the art, the air-fuel ratio in the intake gas introduced into a vehicle engine, for example, can be detected by detecting the oxygen concentration in the engine exhaust. As the detector for detecting the oxygen concentration in the exhaust, there has been known a so-called .lambda. sensor whose electromotive force changes stepwise when the value of the oxygen concentration varies across the value corresponding to the stoichiometric air-fuel ratio. The .lambda. sensor cannot accurately measure individual values of the air-fuel ratio deviating from the stoichiometric value though it can detect whether the actual air-fuel ratio in the intake gas introduced into the engine is larger or smaller than the stoichiometric value.
In vehicles, when the engine is to operate at high power, e.g., when the vehicle is operating under high load or is accelerated, it is preferred that the intake gas be rich, i.e. that the air-fuel ratio be smaller than the stoichiometric value. On the other hand, when the vehicle is cruising at a steady high speed, it is preferred that the intake gas be lean, i.e. that the air-fuel ratio be larger than the stoichiometric value in order to reduce fuel consumption. In order to control the air-fuel ratio to a value different from the stoichiometric value, the actual air-fuel ratio must be accurately detected, and accordingly the .lambda. sensor, which can only detect whether the actual air-fuel ratio is larger or smaller than the stoichiometric value, cannot be used for this purpose.
In Japanese Unexamined Patent Publication No. 57(1982)-76450 and Japanese Patent Publication No. 53(1978)-34077, there have been proposed oxygen concentration detecting devices which can measure individual values of the oxygen concentration. These oxygen concentration detecting devices are both directed to improving said .lambda. sensor which comprises a solid electrolyte body bearing a pair of porous electrodes on opposite sides thereof. The one disclosed in the former publication is a so-called amperometric sensor in which a protective layer is provided on the outer surface of the electrode to be brought into contact with sample gas, to control diffusion of the gas toward the electrode. On the other hand, in the one disclosed in the latter publication, the porous electrodes are poisoned to lower their sensitivity to gradually reduce the overall electromotive force characteristics thereby obtaining linear electromotive force characteristics. However, in both the oxygen concentration detecting devices, linear electromotive force characteristics can be obtained only in the rich region, i.e., the region in which the actual air-fuel ratio is smaller than the stoichiometric air-fuel ratio. Further, the width of the region in which the electromotive force characteristics are linear is narrow; for instance, from 100 to 200 mV, and therefore, the detecting sensitivity of the devices is insufficient for practical use.